1. Field of the Invention
The present invention generally relates to a signal output circuit, and more particularly to a signal output apparatus for switching an output operation of an input signal.
2. Description of the Related Art
Conventionally, an audio amplifying circuit is known to amplify audio signals and output the signals to, for example, a headphone or a speaker.
In order to reduce the noise created upon turning on/off the electric power, the audio amplifying circuit has installed a shut-down function and a muting function.
FIG. 7 is a block diagram showing a configuration of an audio amplifying circuit.
An input signal is transmitted from a signal source 102 to an input terminal Tin of an audio amplifying circuit 101 via a direct current reduction condenser C41. The input signal transmitted to the input terminal Tin is transmitted to an amplifying circuit 111. The amplifying circuit 111 includes a differential amplifying circuit 121, an input resistor R31, a feedback resistor R32, and a switch 122, and is supplied with a reference voltage from a reference voltage generating circuit 112, thereby forming an inverting amplifying circuit.
The amplifying circuit 111 outputs a signal in accordance with the difference between the reference voltage from the reference voltage generating circuit 112 and the input signal transmitted to the input terminal Tin. The amplified signal generated in the amplifying circuit 111 is output to an output terminal Tout, and drives a speaker 103.
The switch 122 is disposed between a junction point of the input resistor R31 and the feedback resistor R32 and an inversion input terminal of the differential amplifying circuit 121, and is switched in accordance with a mute signal transmitted from a controller 104 to a control terminal Tcnt1. The switch 122 switches to connect the junction point of the input resistor R31 and the feedback resistor R32 and the inversion input terminal of the differential amplifying circuit 121 when the mute signal is at a high level, thereby allowing inverting amplification of the input signal, and outputting the input signal from the output terminal Tout.
Furthermore, the switch 122 switches to short circuit the output of the differential amplifying circuit 121 and the inversion input terminal of the differential amplifying circuit 121 when the mute signal is at a low level, thereby creating a mute state for preventing the input signal from being output from the output terminal Tout. Therefore, the switch 122 is switched in accordance with the mute signal transmitted from the controller 104 to the control terminal Tcnt1, thereby allowing control of the input signal transmitted to the differential amplifying circuit 121, and achieving control of the mute function.
Furthermore, the reference voltage generating circuit 112 includes a switch 131, a resistor R41, and a resistor R42. A constant voltage Vdd is applied to the reference voltage generating circuit 112. The constant voltage Vdd is applied to a series circuit including the resistors R41 and R42 via the switch 131. The switch 131 is switched on when a shut down signal transmitted from the controller 104 to a control terminal Tcnt2 is at a high level, thereby allowing the constant voltage Vdd to be applied to the series circuit including the resistors R41 and R42. The switch 131 is switched off when the shut down signal is at a low level, thereby stopping the applying of constant voltage Vdd to the series circuit including the resistors R41 and R42.
The resistors R41 and R42 divide the voltage of the constant voltage Vdd when the switch 131 is switched on, thereby generating the reference voltage, and the reference voltage is supplied to a non-inverting input terminal of the differential amplifying circuit 121. Accordingly, the amplifying circuit 111 assumes an operational state. Here, a condenser terminal Tc is connected to a junction point of the resistors R41 and R42. A condenser C51 is connected at the outside of the condenser terminal Tc and runs to ground. The condenser C51 connected to the condenser terminal Tc absorbs a ripple of the reference voltage and serves to stabilize the reference voltage.
Next, an operation of the audio amplifying circuit 101 is described.
FIG. 8 is a diagram showing an operation of the audio amplifying circuit 101. FIG. 8(A) shows a shut down signal output from the controller 104, FIG. 8(B) shows a switching state of the switch 131, FIG. 8(C) shows a reference voltage supplied to the differential amplifying circuit 121, FIG. 8(D) shows a mute signal output from the controller 104, and FIG. 8(E) shows a switching state of the switch 122.
As shown in FIG. 8(A), the switch 131 is switched from off to on (as shown in FIG. 8(B)) when the shut down signal turns from a low level to a high level at time t10. By switching the switch 131 on, the reference voltage is generated by the resistors R41 and R42. Here, as shown in FIG. 8(C), the reference voltage is gradually increased due to the externally connected condenser C51, and reaches the predetermined level at time t11. When the reference voltage reaches a predetermined level at time t11, a shutdown state of the differential amplifying circuit 121 is released, and becomes an operational state.
The controller 104 counts for a predetermined time from time t10, and outputs the mute signal at time t12 being the time that the predetermined time has elapsed, as shown in FIG. 8(D). As shown in FIG. 8(E), the switch 122 of the amplifying circuit 111 is switched on by the mute signal, thereby releasing the mute state of the input signal, amplifying the input signal in the amplifying circuit 111, and transmitting the amplified input signal to the speaker 103.
Accordingly, in the conventional example, the generation of the reference voltage in the reference voltage generation circuit 112, the operation of the amplifying circuit 111, and the shutdown function are controlled based on the shutdown signal from the controller 104. Furthermore, the mute function of the amplifying circuit 111 is controlled based on the mute signal from the controller 104.
In the conventional audio amplifying circuit, the shutdown signal and the mute signal are to be input separately into an integrated circuit. This, therefore, increases the number of external pins of the integrated circuit, and makes size-reduction difficult.
Accordingly, in order to reduce the number of external pins, an audio amplifying circuit for controlling both the shutdown function and the mute function in accordance with the level of the shutdown signal is proposed, as disclosed in U.S. Pat. No. 5,642,074.
In the conventional audio amplifying circuit, the shutdown function and the mute function are controlled by separately transmitting a shutdown signal for controlling the shutdown function and a mute signal for controlling the mute function from an external controller. Therefore, the shutdown signal and the mute signal are required to be generated at the external controller, and control of the timing for generating the signals is required. Accordingly, this is a processing burden for the controller.
Furthermore, in controlling both the shutdown function and the mute function in accordance with the level of the shutdown signal, it is difficult to accurately manage the timing of the control of the shutdown function and the control of the mute function. There are also some cases where noise is output during the amplification rise time.